Historically as well as today, the transmission of digital data in both public and private networks relies upon the use of modems. Modems are modulators/demodulators designed for transmission of incoming data streams over analog facilities such as a conventional 300-3400 Hz analog telephone line. Modems were devised because it was more economical to use the existing analog telephone network rather than constructing new, separate digital transmission networks.
A telephone link through the switched telephone network is made up of numerous telephone lines once the link is established via call set-up. Although the electrical characteristics of the individual lines may be known with some certainty, it is difficult to predict the overall electrical behavior of the complete circuit, including the end systems with the embedded modems, because of the myriad of paths that may be selected basically at random by the network routing algorithms during set-up of the link. One reason for this difficulty is that significant mismatches may occur between the individual lines as well as between the modems and the lines to which the modems are connected. The overall electrical behavior of the link brought about by such effects as these circuit mismatches, non-linearities and noise--both "gaussian" and impulse type--can cause errors in the received data stream.
A common measure used to describe the occurrence of errors in an end-to-end digital data transmission system including the modems as link terminations is the Bit Error Rate (BER). In practice, the BER is estimated by counting the number of bits received in error (N.sub.E) by a given modem during a specified interval and taking the ratio of N.sub.E to the total number of bits received (N.sub.R) during the interval, that is, BER=N.sub.E /N.sub.R. For example, a BER of 0.5.times.10.sup.-7 would indicate an error for every two million bits received by the given modem.
The BER, besides being a very meaningful measure of performance of the overall system, is an excellent maintenance and diagnostic parameter since degradation which may affect the quality of the transmitted data stream will also have a measurable affect on the BER.
The usual procedure for measuring the BER is to transmit over the end-to-end connection a so-called maximal length, pseudo-random binary sequence generated by an n-stage shift register circuit, or by storing the equivalent sequence in a storage device and emitting the stored bits serially at a given clock rate. A maximal length sequence is expressed by the relation (2.sup.n -1). Typically, sequences of lengths 63, 511 and 2047 bits have been used for low-speed services such as voice-band data. As sequence lengths grow longer, the likelihood of detecting malfunctions associated with specific data patterns increases. The sequence length is selected as a trade-off between the accuracy of the approximation N.sub.E /N.sub.R as representing the true error probability, and the complexity of the alignment, i.e. "block synchronization," in the error detector. Alignment refers to the need to synchronize the received data stream with the test pattern so that errors may be identified bit-by-bit.
Most BER tests are accomplished off-line, that is, there is not transmission of actual data taking place during the BER measurement. In fact in these situations, a so-called Bit Error Rate Tester (BERT) is substituted for the real digital data source. If the end systems are geographically separated as is the case in an actual working or field environment, then two independent BERTs are substituted for each of the end system data sources and the testing can then be accomplished in the full-duplex mode, that is, propagation of data can occur in each direction of transmission simultaneously with uncorrelated data. It is also possible to measure the BER using just one BERT if the far-end modem, that is, the one not having a BERT connected to it, is placed in a "loop-back" mode wherein the modem's data receive and transmit ports are bridged together. This simulates full-duplex testing except that the data streams are correlated.
During the exercising of new modem designs, it is usually the case that the end systems, including the modems and a simulated version of the network link, are co-located in a laboratory environment. Prior art arrangements have merely treated this laboratory situation in a straightforward manner as being an less complex case of general testing. It has not been appreciated that certain testing circuitry such as I/O devices and peripheral device access to the BER test equipment may be unnecessarily replicated. No prior art arrangements have capitalized on the fact that the two systems are co-located to realize a reduction in duplicated circuitry along with concomitant reduction in the cost and increased reliability of the test equipment.